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Gear Friday – Back Button Focus

Canon DSLR cameras like the EOS 50D and 5D Mark II have an option to change the way auto focus is activated. This setting lets you customize the camera so that auto focusing is initiated by pressing the rear “AF-ON” button with your right thumb instead of by half pressing the shutter button. “Back-Button Focus” as its called, offers several advantages such as making it easier to lock focus, making it easier to override auto focus with lenses that provide full-time manual focus and making it possible to switch between focus lock and focus tracking when in AI Servo mode.

Activating “Back-Button Focus” is done on the EOS 50D and 5D Mark II by changing C.Fn IV-1 to one of the settings shown below. I’ll attempt to explain what each of these mean since they definitely seem a little confusing at first.

0: Metering + AF start
This is the default setting where you activate the camera’s meter and AF by pressing the shutter button half-way down OR by pressing the rear “AF-ON” button.

1: Metering + AF start / AF stop:
Again, you activate the camera’s meter and AF by pressing the shutter button half-way down but now, pressing the rear “AF-ON” button locks the focus. Focus is unlocked by removing your thumb from the “AF-ON” button. This is very useful when shooting in “AI Servo” mode when you need to switch from “AI Servo” to “One Shot” mode back and forth. (Hint: This is the setting I use most often for birds in flight)

3: AE Lock / Metering + AF start:
Auto focus is activated solely by pressing the “AF-ON” button. The difference between this setting and option 2 is that when you press the shutter button half-way, your exposure is locked and won’t change until you pull your finger off the button entirely. If you shoot a burst of pictures in any auto exposure mode, the exposure setting used for the first shot is used for each subsequent shot.

4: Metering + AF start / Disable
This is the same as the first option except the rear “AF-ON” button is completely disabled. I suppose this protects you from accidentally pressing the “AF-ON” button (Hint: Dumb setting since this rarely happens).

17 thoughts on “Gear Friday – Back Button Focus”

Hi Jeff, I stumbled upon your web site while searching for “back button focusing on the Canon 7D.” I just switched to the 7D from the Rebel XTi. The 7D’s custom functions are a little different than the one you so clearly outlined above. I am trying to have the metering on the shutter button (pressing 1/2 way) and put the focusing alone (without metering) to the back button (I’m assuming the AF-ON button. However, I am not sure how to assign ‘focus only’ to the back button vs having it focus and meter. On the Rebel I had the * button as my focus button, although there wasn’t an AF-ON button. Thank you very much.
Sincerely,
Laurie

Thanks for reading. If you look on page 218 of your 7D manual it shows how to assign different functions to the various controls. It’s a bit more complicated than on previous Canon cameras but if you activate the AF-ON button it “should” remove all AF functions from the shutter. A half-press should still meter but not focus.

Jeff, heard about “back button focus” from the Gerlach web site. Found your web site using a Google Search to find out more. Like what I see, so I’ll become a regular.

I understand what BBF does, but I don’t understand the problem it is trying to solve. Maybe I am just dumb and happy using the shutter release button pushed part way, and I don’t seem to have a problem … or do I?

Looking forward to understanding more about this technique and what problem it overcomes.

Mind you, the camera is VERY new so I should run some tests. But when zooming in to an image from the 7D and looking at the sharpest point, it still seems a bit soft. The two images that seemed most likely to be good “tests” were both shot on a tripod, one with the 100mm 2.8 and the other with the 50mm 1.4. Shutter speeds and ISO I’m not certain of as I type this. I know 100mm was at 1/45 at f4.5. ISO was likely set to 200 in both cases. As I zoom in and move around the picture looking for the focus point, even the sharpest detail still feels a bit soft. Raw conversion is done via the PhotoShop plug-in. I think I need to run some more tests before making a final determination. Any suggestions would be very much appreciated.

Slightly off topic – but just slightly. I’m wondering what your take is on the image SHARPNESS of the 7D? I’m a long-term Nikon-only shooter and I finally got tired of Nikon lagging behind Canon in technology (not to mention being about 20% more expensive), so I sold off all my Nikon gear and went with all Canon gear – including the 7D. I’ve only just received the camera and lenses and I’m a bit worried because no matter how perfectly the image is focused, it still seems to have an overall “softness” when I zoom into it in PhotoShop or using the camera controls. This is at the highest res jpeg and RAW files. Please tell me I’m doing something wrong and that this is not an issue with the sensor.

Image “softness” can be due to any number of factors including lens sharpness, camera movement and raw conversion. I highly doubt it has anything to do with the sensor itself but there is a new firmware out for the 7D that fixes an image ghosting issue. Tell me more about your setup and I may be able to suggest a few things.

I’m trying to get the hang of shooting with wide open apertures like f1.4, f2, etc. however I am having problems achieving consistent focus; I’m simply too slow to get the focus right and get the exposure right (I shoot manual mode). I shot some some photos at a wedding as a guest and some turned out really well but I’d like to be better. Which of these settings do you recommend I use? I have a Canon 5D.

Thanks for reading. When you shoot wide open, the depth of field is very narrow and it may seem like your shot is not in perfect focus. A good rule of thumb for shooting an individual portrait is f/5.6 to ensure that the entire face is in focus. If you shoot at f/2 or wider, you may want to select a single focus point and focus on either eye of your subject. I’d also recommend shooting in aperture priority (Av) mode.

Thanks for reading. The settings for back button focus on the new 5D MK II are exactly the same as on the 50D. The 450D / XSi is a little different since this body lacks the dedicated focus button on the back. You can switch the (*) button to AF lock using C.Fn-10 shutter/AE lock button however.

Thanks for the tips, Jeff. I’ve been waiting a long time for someone to help explain all those settings.

One setting I wish they had is to let the shutter button activate auto focus in Live View mode. I don’t understand why I’m forced to use two buttons to take a simple snapshot in Live View. Doesn’t make sense.